Examples of micro-optical electromechanical system (MEOMS) release structures include membranes and cantilevered structures. Both of these structures are used in a spectrum of optical applications. For example, they can be coated to be reflective for a spectral band of interest and then paired with another mirror to form a tunable Fabry-Perot (FP) cavity/filter. They can also be used as stand-alone reflective components to define the end of a laser or interferometer cavity, for example.
The structures are typically produced by depositing a device layer over a sacrificial layer, which has been deposited on a support. This sacrificial layer is subsequently etched away or otherwise removed to produce the release structure in a release process. In some examples, the device layer is a silicon compound and the sacrificial layer can be polyimide or oxide, for example.
Typically, release structure deflection is achieved by applying a voltage between the release structure and a fixed electrode on the support structure. Electrostatic attraction moves the structure in the direction of the fixed electrode as a function of the applied voltage. This results in changes in the reflector separation in case of an FP filter, laser, or other optical cavity.
The performance of the MOEMS device typically is affected by the mechanical stability of the release structure and its susceptibility to interference from its immediate environment. It is not uncommon to install the device in a hermetic package with a temperature controller such a thermoelectric cooler. These safeguards help to limit long-term changes due to aging and thermally induced changes in the device.
Stability, however, can be affected by other factors, such as signal power levels. Semiconductor materials, such as silicon, can absorb light. Even photons with energy below the intrinsic bandgap, such as in the infrared communication wavelengths, can be absorbed through surface states, defects, excess dopants, or two-photon absorption. This absorption can give rise to electron-hole pair generation, which impacts the electric fields surrounding the release structure. This effect can be aggravated by dielectric-based optical coatings because of charge trapping at dielectric interfaces. High powers can also yield temperature gradients.
The effects can be observed in, for example, a tunable Fabry-Perot filter, when tuned to the signal frequency. The filter transforms from essentially reflecting all of the incident light to becoming transparent to it, thereby causing a large change in the surface potential, which moves the optical passband either away from or in the direction of the desired tuning position. The result is a power-dependent passband shape that makes optical parameters, such as the optical signal to noise ration (OSNR), difficult to measure at higher power levels. Another effect of the absorption is heating that gives rise to a power-dependent shift in passband frequency as the filter is thermally expanded by the incident light.
These effects have been observed at mid-power ranges, e.g., xe2x88x9215 dBm to 0 dBm, and high-power, e.g., 0 dBm to 20 dBm, and higher. Such power levels are commonplace with the advent of the erbium-doped fiber amplifier (EDFA) and can be encountered by a receiver filter in a preamplified optical receiver, for example. The result can be the degradation of performance of a tunable MOEMS optical filter.
The present invention is directed to a MOEMS device and corresponding fabrication process in which absorbing material along the optical axis of the device is removed. The result is a suspended optical coating, such as a dielectric thin film mirror stack. Such optical coatings can have very low absorption. Thus, the invention can materially lower the net absorption in the devices, and thereby improves performance by, for example, reducing signal power dependencies.
In general, according to one aspect, the invention features a process for fabricating a micro-optical electromechanical system device. The process comprises depositing an optical coating that is supported by a device layer. A sacrificial layer is removed to form a release structure in the device layer along the optical axis.
Depending on the implementation, the optical coating can be deposited before or after removal of the sacrificial layer to form the release structure.
Finally, a device layer port is formed by removing a portion of the device layer opposite the optical coating in a region around the optical axis. Thus, the optical signal propagating along the optical axis does not have to pass through the material of the device layer or at least less device layer material.
In the described applications, the optical coating is a high reflectivity coating, having greater than 90% power reflectivity to thereby form a mirror structure.
In the preferred embodiment, the optical coating is formed using thin film technology, in which the alternating layers of high and low refractive index materials are deposited on the device layer. The thickness of the layers is related to the wavelength of light that system is intended to handle. Typically, the layers are about one-quarter of a wavelength in thickness, with six or more layers being common for an highly reflecting (HR) coating.
In the typical implementation, the sacrificial layer is removed via an etch process. This sacrificial layer is commonly sandwiched in a region between the device layer and a support. The support can be silicon handle wafer material. In such cases, it is not uncommon to fabricate a support optical port through the wafer material.
According to one implementation, an etch stop layer is first deposited on the device layer, then the optical coating is deposited on the etch stop layer. Then, when the device layer port is formed, an etch is performed through the device layer to the etch stop layer, which can then be later cleared away in another etch process.
In general, according to another aspect, the invention features a MOEMS device. This device comprises at least a first mirror structure and a second mirror structure. These mirror structures define an optical cavity. Further, a support is provided and a deflectable structure on the support that holds the first mirror structure. This deflectable structure is located on an external side of the first mirror relative to the optical cavity. The deflectable structure has an optical port in a region around the optical axis of the optical cavity.
The location of the deflectable membrane structure the outside of the cavity prevents intracavity losses, but renders the device susceptible to instability in operation. The magnitude of light transmitted through the material can change dramatically as the filter""s resonance is tuned on and off a signal of interest. This is avoided in the present invention by providing the optical port through the deflectable structure.
The first and second mirror structures preferably comprise dielectric thin film coatings. The support can be fabricated from silicon wafer material. The insulating layer is preferably provided between the support and the deflectable structure. This insulating layer, in one embodiment, functions as an electrostatic cavity spacer. The optical port is typically sized in response to the mode field diameter, so that absorption is minimized.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.